Installation for producing a fiber optic cable component

ABSTRACT

The optical fiber paid out from a reel passes through an extrusion head, then through a cooling vat, the sheath extruded around it being pulled by a withdrawal device. The sheath then passes through a second vat, when the stranding element is withdrawn by an auxiliary withdrawal device under as low a tension as possible. In the second vat, the stranding element is reduced to a temperature lower than the ambient temperature so that contraction of the sheath ensures the excess length of the optical fiber.

This is a division of application Ser. No. 06/804,119 filed Dec. 3,1985, now U.S. Pat. No. 4,772,435.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to cable manufacture, and more particularly to aninstallation for producing a fiber optic stranding element, of the typehaving at least one payout reel for a light-waveguide component, anextrusion head, a cooling device, and a device for receiving a finishedstranding element. The invention also relates to further the strandingelement thus produced.

During the manufacture of fiber optic cable, the fiber component orcomponents must be longer than the surrounding sheath or sheaths. Onemajor problem which thus presents itself in such manufacture is that ofcontrolling this excess length of the fiber waveguide component(s) ascompared with the sheath component(s). The light-waveguide(s), formed offibers exhibiting a certain fragility, must be loose in their protectivesheath; and care must be taken that the deformation to which thestranding element will be subjected when it is wound on the takeupreels, then during handling and laying, is not apt to damage the fragileconstituents of the fibers. European Patent No. 0 015 425, for example,describes means which may be provided for ensuring that light-waveguidecomponents are loose within their protective sheaths.

It is an object of this invention to provide an improved installationfor producing fiber optic cable whereby the light-waveguide componentsare given the necessary excess length within the sheath or sheaths.

A further object of this invention is to provide an improved method ofproducing such fiber optic cable in order to achieve this excess length.

To this end, in the installation according to the present invention, ofthe type initially mentioned, the payout reel or reels are provided withbrakes whereby the tension of the guide component or components can beregulated, the cooling device comprises a first vat adjusted for coolingthe stranding element to a first temperature T₁, a main withdrawal meansfor pulling the stranding element through the extrusion head and thefirst vat, a second vat adjusted for cooling the sheath to a secondtemperature T₂ lower than the temperature T₁, and auxiliary withdrawalmeans capable of pulling the stranding element through the second vatwith as low a tractive force as possible, and the receiving device iscapable of receiving and driving takeup reels for storing the strandingelement at the exit of the auxiliary withdrawal means.

In the method according to the present invention, likewise of the typeinitially mentioned, the cooling is effected in two successive stages,the waveguide component and the sheath are subjected to predeterminedtensile stresses during the first stage of cooling, whereas during thesecond stage, the waveguide component is slack and the sheath issubjected to as low a tensile stress as possible, and the coolingtemperature obtained during the second stage is lower than that obtainedduring the first stage.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described in detailwith reference to the accompanying drawings, in which:

FIG. 1 is a diagram of the various elements of the installation,

FIG. 2 is a diagrammatic detail view on a larger scale, showing theposition of the stranding element upon entering the main withdrawaldevice, and

FIG. 3 is a view similar to that of FIG. 2, showing the position of thestranding element upon leaving the main withdrawal device preparatory toentering the second cooling vat.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the installation illustrated in FIG. 1, a light-waveguide 1,hereafter to be called "optical fiber," is borne by a payout reel 2 andpaid out at a tension Ff, maintained at a value ensuring that the fiberremains taut all the way to a main withdrawal device, or extractor, 5.Tension Ff is maintained by means of a magnetic brake 2a acting uponreel 2 and may vary between 20 and 100 grams.

Restrained by payout reel 2, optical fiber 1 enters an extrusion head 3fed from a conventional extruder, so that a sheath 11 of plasticmaterial forms around fiber 1. Upon leaving head 3, therefore, fiber 1is encased in sheath 11, this operation being carried out in such a waythat fiber 1 remains loose within sheath 11. The latter then enters afirst cooling vat 4, supplied with water, for instance, and adjusted sothat the encased fiber is kept at a predetermined temperature T₁. Thelength of vat 4 is calculated as a function of the rate of travel ofsheath 11 so as to stabilize the temperature thereof, even at themaximum output speed of the line.

Upon leaving vat 4, the temperature of sheath 11 is equal to T₁. Itpasses onto main extractor 5, consisting, for instance, of alarge-diameter pulley over which sheath 11 makes several turns. Mainextractor 5 is rotatingly driven at a constant speed to pull fiber 1through extrusion head 3. Extractor 5 is kept at temperature T₁. In oneparticularly advantageous embodiment of the invention, extractor 5 maybe accommodated within vat 4.

After leaving main extractor 5, sheath 11 enters a second cooling vat 6designed to reduce its temperature to T₂, which is not only lower thanT₁ but even lower than the ambient temperature at which the finishedstranding elements received on takeup reels 10 are to be stored. To pullsheath 11 through vat 6, there are provided an auxiliary withdrawaldevice, or extractor, 8, capable of storing a certain length ofstranding element, and two pulley assemblies 7 and 9, one disposedbetween vat 6 and auxiliary extractor 8, the other between the latterand a winder 10 bearing the takeup reel. Pulley assemblies 7 and 9 aredesigned to ensure that the stranding element is moved at a tension Ftwhich is as low as possible. It will be realized that, under theseconditions, the transition from temperature T₁ to temperature T₂ causessheath 11 to contract, thus giving waveguide 1, accommodated withinsheath 11, a certain excess of length. At the entry into extractor 5(see FIG. 2), waveguide 1 is held taut within sheath 11 as a result ofthe tensile stress caused by brake 2 a, which may be a hysteresis brake,for example. Upon leaving extractor 5, on the other hand, waveguide 1 iscompletely relaxed within sheath 11, as shown in FIG. 3.

It follows from the foregoing that if sheath 11 has an outside diameterD and an inside diameter d, and if the light-waveguide has a diameterdf, provision is made for an appreciable difference between diameter dand diameter df, even in the case where a single waveguide isaccommodated in the sheath.

However, it should be clearly understood that the installation andmethod described above are equally applicable to the case where aplurality of optical fibers 1 are situated together within the samesheath 11. The same installation and method are likewise applicable whenseveral groups of optical fibers, each jacketed within a protectivesheath, are encased simultaneously in an outside sheath intended toprotect the cable as a whole.

Temperature T₁ corresponds to a partial cooling of sheath 11 andwaveguide 1. The value of this temperature will be determined as afunction of the size and the materials used. It will preferably behigher than the ambient temperature. The decisive effect for obtainingthe desired result is that, after the contraction undergone by sheath 11during the second cooling stage, at as low a tension as possible, thelength of sheath 11 is fixed on auxiliary extractor 8 and takeup reel10, so that the excess length of the light-waveguide component(s) ispreserved. This excess length is thus determined by the differencebetween the cooling temperatures (T₁ -T₂). Hence, cooling of thestranding element is carried out in two stages, viz., a first coolingstage carried out at a predetermined tension, whereby the optical fiberis pulled through the extrusion head, then a second stage carried outwith the stranding element in a state of zero tension, so that thelength can contract freely. This free contraction gives the opticalfiber a slight excess of length within its sheath, thus allowing thestranding element to be subsequently manipulated without any risk ofdamage to the fiber.

What is claimed is:
 1. An installation for producing a fiber opticstranding element formed of at least one light-waveguide componentloosely encompassed in at least one sheath, said installationcomprising:at least one payout reel for holding at least onelight-waveguide component, an extrusion head located downstream of saidat least one payout reel for forming at least one sheath looselyencompassing said at least one light-waveguide component to form saidstranding element, braking means associated with said at least onepayout reel for regulating the tension of said at least onelight-waveguide component, cooling means for cooling said at least onesheath, said cooling means being located downstream of said extrusionhead and including a first vat for cooling said at least one sheath to afirst temperature T₁ and a second vat for cooling said at least onesheath to a second temperature T₂ lower than said temperature T₁, mainwithdrawal means for simultaneously pulling said at least onelight-waveguide component through said extrusion head and said strandingelement through said first vat, auxiliary withdrawal means for pullingsaid stranding element through said second vat with as low a tractiveforce as possible, at least one takeup reel disposed downstream of saidauxiliary withdrawal means for receiving and storing said strandingelement, and means for driving said at least one takeup reel.
 2. Aninstallation for producing a fiber optic stranding elementcomprising,payout means for paying out a continuous light-waveguideelement, braking means associated with said payout means, extrudingmeans located downstream of said payout means for extruding a continuoussheath onto said light-waveguide element and loosely encompassing saidlight-waveguide element, a first cooling vat downstream of saidextruding means for cooling said sheath and said light-waveguide elementto a first temperature T₁, first pulling means for pulling said sheathand said light-waveguide element together through said extrusion meansand said first vat, said first pulling means cooperating with saidbraking means for keeping said light-waveguide element under apredetermined tension, a second cooling vat located downstream of saidfirst pulling means, for cooling said sheath and said light-waveguideelement from the first temperature T₁ to a second temperature T₂ lowerthan said temperature T₁, and second pulling means located downstream ofsaid second cooling vat and arranged for pulling said sheath and saidlight-waveguide element together through said second cooling vat whilekeeping them under a tension force as low as possible, thus allowingsaid sheath to shrink when cooling.